1. Field of the Invention
The present invention relates to a method for measuring particle size distribution in masses of granular substances, such as various raw materials for making iron, cereals, etc., and more particularly to a method of measuring particle size distribution continuously during transportation of the particles. The invention also relates to an apparatus to carry out particle size measurement according to the method.
2. Description of the Prior Art
As is well-known, it is important to know the particle size distribution in various raw materials to be charged into a blast furnace just before they are charged in order to stabilize the operating conditions of the furnace, because the particle size distribution in the raw materials plays an important role on the permeability of air through the furnace. Furthermore, is is necessary for judging the quality of cereals to know the particle size distribution.
The measurement of the particle size distribution of masses of granular substances has, hitherto, been done in a way exemplified by how measurement has been carried out on the raw materials used for iron making. Midway along a conveyor belt for transporting these materials is provided a sampling device, and samples are taken from the raw material which is being conveyed on the conveyor belt continuously one by one at appropriate time intervals. Each sample is divided into 4 to 5 fractions using sieves and the weight of each fraction is measured by a weighing device. The particle size distribution of the raw material is then determined from the relative weight of each fraction of the sample in relation to the whole sample. This method of measurement has several disadvantages. Both the sampling device and the weighing device must be large, because it is necessary to make each sample large in order to make the results accurate and useful for the large amount of raw material consumed in a blast furnace. Further, the time interval for sampling, or in other words, the sampling period, is usually rather long, such as 30 minutes to 1 hour, due to the fact that the time necessary for the dividing up of a sample is rather long.
This method makes the validity of each sample for the total amount of material used in a furnace uncertain, and it has thus been difficult to get reliable data on the particle size distribution by this prior art method. Moreover, since it takes more than 1 hour to determine the size distribution of a sample after taking it from raw material being conveyed, any proper feedback of these results, to adjust the particle size distribution, is too late to properly adjust the particle size distribution effectively. This is a serious disadvantage of the prior art method.